impedance (was negative crown_

[Doug Richards]

Frank,
Exactly my point!  Here I thought I was going to show how much I didn't
know.  In mechanical optimization of disk drives, we like to talk about
things we can measure and then change.  If you can't measure it, all the
beautiful words describing the concept are just that.  Why not switch gears
and talk of things you can measure and then optimize?  I'm a finite element
modeler so I do lots of computer simulations to show trends, but I always
have to talk in terms that  the lab guy can go out and test the structure
and corroborate my predictions.  He measures mechanical properties like
displacement, velocity and acceleration and can display them in terms of
real/imaginary or magnitude/phase as a transfer function.  These are the
tools we use to measure and describe vibrating mode shapes, resonant
frequencies, internal damping, transient shock response and so on.

So back to pianos...

What DID Del do to kill the Killer octave (without bringing up impedance)? 

doug

> -----Original Message-----
> From:	Frank Weston [SMTP:klavier@annap.infi.net]
> Sent:	Wednesday, December 30, 1998 5:55 AM
> To:	pianotech@ptg.org
> Subject:	Re: impedance (was negative crown_
> 
> > So how would YOU measure the mechanical impedance of a structure?  Is it
> > just the real and imaginary part of a transfer function?  I still don't
> know
> > how to measure mechanical impedance...
> > 
> > doug
> 
> Why Doug!  I think you've just hit the nail on the head (which by the way
> can be a good example of mechanical impedance match/mismatch).  While
> mechanical impedance is a convenient concept, it is very nearly impossible
> to calculate in complex mechanical system like a piano given the resources
> at the disposal of those who design pianos.  From what we read here, we
> know it's a complex number and a function of inertia and "springiness",
> and
> that the imaginary component drives the affect with respect to time.  What
> we don't know is how to calculate it.  In fact, all the computers on this
> list running in parallel until the end of the millennium would not have
> the
> time or power to even dent the problem.  A more realistic approach, and
> the
> one adopted by all who design pianos is the empirical one, which is to
> build a piano, see what happens, make a change, see what happens, make
> more
> changes, etc. 
> 
> Want a demo of just what mechanical impedance is?  Take a hammer and a
> four
> foot 2x4 and a nail.  Lay the board flat on top of your workbench.  Pound
> in the nail.  Easy, right?  Now take the board and extend it about two
> feet
> off the edge of your bench, again, flat side down.  Clamp the other end to
> the bench.  Now try to pound a nail into the board from the top near the
> extended end.   Much more difficult.  Now drive the nail into the butt end
> of the board at the extended end.  Much easier.  Now rotate the board 90
> degrees and clamp as before.  Drive a nail from the new top.  Easier, but
> not as easy as other approaches.  Try different sized nails, different
> hammers, different angles and extensions.
> 
> What you will have experienced is a range of mechanical impedance matches
> from good to poor.  In this simple case, it is even possible to calculate
> impedance, but why waste the time?  It's much easier to just position the
> board and drive the nail, no matter what impedance might be calculated,
> the
> nail will be just as hard to drive.  Draw the piano analogy on your own.
> 
> Frank Weston -  Why yes, I am a rocket scientist.
> 
> 
> 
>  
> ----------
> > From: Doug Richards <Doug.Richards@quantum.com>
> > To: pianotech@ptg.org
> > Subject: RE: impedance (was negative crown_
> > Date: Wednesday, December 30, 1998 2:52 AM
> > 
> > 
> : birketts@wright.aps.uoguelph.ca